(ABCNEWS.GO) — Sen. Tim Johnson, D-S.D., is set to announce Tuesday that he will not seek re-election, according to two sources, becoming the seventh incumbent to retire after next year.

Johnson, who battled back after suffering a life-threatening brain hemorrhage in 2006, is chairman of the Banking, Housing and Urban Affairs Committee. But in political terms he is something else: A Democrat serving in a deeply conservative state. His retirement gives Republicans another open seat to pursue in their quest to win control of the Senate.

He has scheduled an announcement at 3 p.m. Tuesday at the University of South Dakota in Vermillion, his alma mater, to disclose his plans. One Democratic source in South Dakota and another official in Washington, both of whom spoke on condition of anonymity to avoid upstaging the official announcement, confirmed the decision.

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